1. Field of the Invention
The present invention relates to a technology for generating a projection image which is an image formed of image information on a plurality of visual lines viewing a structure in a three-dimensional medical image from a predetermined viewpoint being projected on a predetermined projection plane.
2. Description of the Related Art
Recently, with the advancement of modalities, such as multidetector-row CT (MDCT), high quality three-dimensional image data have become available. In image diagnosis using such image data, virtual or pseudo three-dimensional subject images have been used more frequently, as well as high resolution cross-sectional images. Further, the extraction of organ structures from three-dimensional medical images using image recognition technologies are also performed, and it has become possible to generate virtual or pseudo three-dimensional images from three-dimensional medical images of well distinguished structures. Then, the images so generated are used in the planning and simulations prior to surgery and navigations during surgery.
As for the method of generating a virtual or pseudo three-dimensional image, a method using rendering technology, such as surface rendering or volume rendering is known. In particular, an image obtained by perspective projection (central projection) from a virtual viewpoint inside of a subject is called a virtual endoscopic image, which is an image as if observed by an endoscope.
Technologies for supporting endoscopic surgery using such virtual endoscopic images are proposed.
For example, U.S. Patent Application Publication No. 20020128547 describes an apparatus that detects a position of an endoscope by a sensor and generates, with the detected position as the viewpoint, a virtual endoscopic image having a wider field angle than the endoscope, and displays a real endoscopic image captured by the endoscope and the virtual endoscopic image in a superimposing manner.
Further, Japanese Unexamined Patent Publication No. 2005-021353 describes an apparatus that detects a position of an endoscope in real time, generates a virtual endoscopic image having the same field of view as that of the endoscope and visualizing a vascular arrangement within the field of view, detects a position of a treatment tool used in the endoscopic surgery in real time, generates a composite image by combining an image representing the treatment tool to the virtual endoscopic image at the position of the treatment tool, and displays the composite image and a real endoscope image.
In an endoscopic surgery, the visual line direction differs from the operation direction of the treatment tool. This makes it difficult, even by observing, in perspective, a lesion in an organ using virtual endoscopic images from endoscope viewpoints described in the patent documents described above, to understand the operation direction of the treatment tool, i.e., the state in an approaching path of the treatment tool towards a lesion in the images. For example, for a virtual endoscopic image in which the visual direction VL corresponds to the center of the field of view, as schematically illustrated in FIG. 7, it is difficult to understand a position where the treatment tool enters into an organ. Although it is possible to observe other structures present in the organ, such as a blood vessel and the like, to be taken into account for the surgery by translucent processing, it is difficult to understand whether or not such a structure is present in the path of the treatment tool from the entry position to the lesion.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an apparatus and method for generating a projection image that allows easy understanding of the state of an approaching path of a treatment tool for an attention structure masked by another structure, such as a lesion in an organ. It is a further object of the present invention to provide a computer readable recording medium on which is recorded a projection image generation program of the present invention.